1. Field of the Invention
This invention relates to a toner and a method for the production thereof. More particularly, this invention relates to a toner for developing electrostatic latent images to be formed as by the electrophotographic method, electrostatic recording method, and electrostatic printing method and a method for the production of the toner.
2. Prior Art
For the operation of the photographic method, the heat roller fixing method has been finding growing acceptance as a means for permanently fixing in situ a toner image formed on an image receiving sheet.
This method is ideally suitable for an electrophotographic copying device because it utilizes the pressure contact to be established between the surface of a heat roller and the image surface of a sheet subjected to image fixation and, therefore, excels in the thermal efficiency with which the toner image is thermally fused to the sheet and permits quick fixation. In spite of the advantage mentioned above, this method suffers from a serious problem of entailing the phenomenon of offset. The term "offset" means the phenomenon that, in the process of image fixation, part of the toner forming the image adheres to the surface of a heat roller and this toner transfers onto the next sheet subjected to image fixation and consequently smears the image to be formed thereon.
For the purpose of preventing the phenomenon of offset, such measures as applying silicon oil or a similar substance as a release agent to the heat roller, incorporating such a low melting wax as low molecular polypropylene, low molecular polyethylene, or paraffin wax as a release agent in the toner, and widening the range of molecular weight distribution thereby heightening the cohesive force of molten toner particles have been adopted besides selecting the kind of material for the roller.
In recent years, the desirability of improving the heat roller fixing method in terms of power consumption and speed of fixation has been finding growing recognition. It has been held that the measure of lowering the magnitude of Tg and that of melt viscosity of the toner binder resin is effective in attaining this improvement.
A decrease in the magnitude of Tg of the toner binder resin, however, forms a cause for lowering the stability of storage to resist the phenomenon of blocking or lowering the flowability of toner particles and a decrease in the magnitude of melt viscosity results in rendering the phenomenon of offset more conspicuous.
For the sake of solving these problems, JP-A-2-5,071, for example, proposes a method which comprises adding to a toner composition the oligomer of a crystalline acrylic ester or a crystalline methacrylic ester containing as a component unit thereof such a monomer as stearyl acrylate or stearyl methacrylate. U.S. Pat. No. 4,514,487 likewise discloses a toner which is obtained by polymerizing a polymerizing monomer as a component of a binder resin in the presence of the oligomer of stearyl acrylate or the oligomer of a copolymer containing stearyl acrylate.
Indeed the oligomer of such a crystalline acrylic ester or crystalline methacrylic ester as mentioned above is capable of lowering the minimum fixing temperature of the produced toner because it has a low melting point and a low melt viscosity. When this oligomer is added to the toner binder resin and melted and mixed therewith during the production of the toner, since the difference in melt viscosity between the toner binder resin and the oligomer is large, the oligomer is not thoroughly dispersed in the toner binder resin and the domains of the oligomer present in the matrix of the toner binder resin are relatively large.
When the domains of the oligomer dispersed among the toner particles are large as mentioned above, the disadvantage arises that the resistance offered to the occurrence of the phenomenon of offset owing to the presence of the oligomer is not thoroughly manifested, the flowability of toner particles and the resistance to the phenomenon of blocking are degraded, and the degradation of the flowability retards the initiation of charging of the toner.